The two Raleigh men were unfamiliar with downtown and said they thought the Century Post Office, which houses a few federal offices, was the Federal Courthouse.
The building has a security checkpoint for visitors to the federal offices upstairs, but otherwise the post office lobby is open to the public.
Investigators say Williams had a fake police ID, was wearing a turtleneck with the word "police" on the collar and a police vest.
He told the men he was an US Marshall and could sell them cars that had been seized from criminals --a BMW 5 series and a Honda Accord.
He apparently collected $3,000 in cash from one of the men and more than $6,000 from the other.
A few months after the meeting, the victims said the man they knew as "Manny" still didn't have the cars and refused to return their money. So they went to police.
After a four month investigation, they arrested Williams, who's court records includes a lengthy rap sheet with past arrests for everything from assaults, to firearms violation, to theft by deception.
Raleigh Police investigators say they are trying to find out if this is the only con-job Williams allegedly pulled. If anyone recognizes Williams as someone who may have committed a crime against them, they are urged to contact police.